The linear free-energy relation in the thermodynamics of complex formation. Part 2. Analysis of the formation constants of complexes of the large metal ions silver(I), mercury(II), and cadmium(II) with ligands having ‘soft’ and nitrogen-donor atoms

Abstract

A potentiometric and calorimetric study has been made of the complex formation of Ag^I, Hg^II, and Cd^II with ligands containing ‘soft’ and N-donor atoms. It has been found that linear enthalpy relations exist, and that they mirror quite closely the division of the free-energy data into separate groups for the ‘soft’ and N-donor atom ligands, for the pair of metal ions Ag^I–Hg^II and for Ag^I–Cd^II. It seems that a linear entropy relation also exists, to within experimental error, for the pair Ag^I–Hg^II which has a gradient and intercept that can be rationalised in terms of the Powell–Latimer–Cobble semi-empirical theory of ionic entropy. The entropy contributions are small compared with those of the enthalpy in these predominantly enthalpy-stabilised complexes. The significance of the relatively simple behaviour of the free energies compared with their enthalpy and entropy components is discussed.